A recording/reproducing device for storing a large amount of data, such as an optical disc device, stores information by converting a pulse pattern that changes according to recording information into a pulse pattern that is smaller than a minimum unit of information, changing intensity of a laser beam based on the smaller pulse pattern, focusing the beam having been subject to the change of intensity onto the recording medium and heating the recording medium, so as to change physical characteristics of the medium and form a recording mark.
As density of an optical disc becomes higher, a strict set up is required. For example, it is required to set up a recording condition with respect to each mark length (size of a recording mark in a track direction) or with respect to each combination of the mark length and a space (space between recording marks) length.
Conventionally, jitter has been often used as an evaluation value of the reproduction signal quality of a recorded track for the purpose of setting a recording condition of an optical disk. However, PRML (Partial Response Maximum Likelihood) is being adopted as a data detection method for realizing higher density storage recently. Under such condition, jitter which represents irregularities in a direction of a time base is not suitable as an evaluation value.
Further, a bit error rate of a data detection result that has been obtained by PRML is used as the evaluation value, but this brings about many disadvantages as follows: a large number of sample bits are required upon measurement, and defects caused by flaws of a disk tend to influence the evaluation, and other similar disadvantages are brought about.
In such background, an evaluation method, called SAM (Sequenced Amplitude Margin), by which quality of a reproduction signal is evaluated, is proposed (T. Perkins, “A Window Margin Like Procedure for Evaluating PRML Channel Performance”; IEEE Transactions on Magnetics, Vol. 31, No. 2, 1995, p 1109-1114).
A concept of SAM is described with reference to FIGS. 35 and 36. As an example, the following describes a case where a reproduction signal of a bit sequence that has been recorded on the basis of (1, 7) RLL (Run Length Limited) Coding is detected in PRML, in accordance with PR (1, 2, 1) properties.
As shown in FIG. 35, a reproduction signal waveform in accordance with PR (1, 2, 1) properties with an ideal 1T mark free from any distortion or noise has a 1:2:1 level ratio of samples for a channel clock. For a reproduction signal waveform from a 2T or more mark, the level ratio is obtainable from the superimposition of the reproduction waveform from a 1T mark. For example, the sample level ratio is 1:3:3:1 for the one with a 2T mark, 1:3:4:3:1 for the one with a 3T mark, and 1:3:4:4:3:1 for the one with a 4T mark. An ideal reproduction waveform can be assumed for any given bit sequence. There are five ideal sample levels (ideal sample levels): 0, 1, 2, 3, and 4. Here, for convenience, sample levels are normalized so that the maximum amplitude is ±1. At that time, there are five ideal sample levels: −1, −0.5, 0, +0.5, and +1.
Here, as a technique for specifically realizing PRML decoding, Viterbi decoding is adopted. The Viterbi decoding is described as follows with reference to a trellis diagram shown in FIG. 36. In FIG. 36, S(00), S(01), S(10), and S(11) each represents a different state: for example, the state S(00) means that a previous bit is 0 and a current bit is 0. A line linking a state to the other is termed a “branch,” which represents a state transition: for example, a branch of S(00)→S(01) represents a bit sequence “001”.
In FIG. 36, characters a through f are assigned as an identifier of each branch. An ideal waveform level that is expected in each change in states is added to the characters. For example, because “a” indicates a bit sequence “000”, its ideal level is −1. Because “b” indicates a bit sequence “100”, its ideal level is −0.5. Here, the S(01)→S(10) and S(10)→S(01) branches are missing from the diagram. This is because the bit sequences “010” and “101” cannot occur due to the d=1 (1,7) RLL.
In the trellis diagram, a “path” is formed by connecting continuous branches between the states. To consider all the paths generated after transiting from any one of states to another means to consider all the possible bit sequences. The maximum likelihood path, or the “correct path,” can be determined by comparing the waveform actually reproduced from an optical storage medium with every ideal waveform derived from the paths to find the ideal waveform that is the “closest” to the reproduced waveform, that is, the one with the least Euclidean distance from the reproduction waveform.
A Viterbi decoding procedure based on a trellis diagram will be specifically described. At any given time, there are two paths merging at each of states S(00) and S(11), whereas there is a single path coming in to each of S(01) and S(10). Of the two paths merging at S(00) and S(11), retain the one with a less Euclidean distance between the ideal waveform and the reproduction waveform; this leaves four paths each terminating at a different one of the four states at any given time.
Here, the square of the Euclidean distance between the ideal waveform for a path and the reproduction waveform is termed the path metric. The path metric is calculated by summing up branch metrics for all branches making up the path (the branch metric is the square of the difference between the ideal sample level of the branch and the sample level of a reproduction waveform).
When a sample level of the reproduction waveform at time t is X[t], branch metrics of branches a, b, c, d, e, and f at time t are Ba[t], Bb[t], Bc[t], Bd[t], Be[t], and Bf[t] respectively, and path metrics of survival paths at the states S(00), S(01), S(10), and S(11) at time t are M(00)[t], M(01)[t], M(10)[t], and M(11)[t] respectively, the branch metric is calculated in accordance with the equation (1), and the path metric is calculated in accordance with the equation (2). A process of selecting a smaller path metric from M(00)[t] and M(11)[t] corresponds to determination of a survival path.Ba[t]=(X[t]+1)2 Bb[t]=Bc[t]=(X[t]+0.5)2 Bd[t]=Be[t]=(X[t]−0.5)2 Bf[t]=(X[t]−1)2  equation (1)M(00)[t]=Min{M(00)[t−1]+Ba[t],M(10)[t−1]+Bb[t]}M(01)[t]=M(00)[t−1]+Bc[t]M(10)[t]=M(11)[t−1]+Bd[t]M(11)[t]=Min{M(01)[t−1]+Be[t],M(11)[t−1]+Bf[t]}(Min{m,n}=m(if m≦n):n(if m>n))  equation (2)
When the procedure for determining the survival path is repeated every time the sample values of the reproduction signal waveform are inputted, a path with a greater path metrics is eliminated, so that the number of paths is gradually narrowed into one. This one is regarded as the correct path, so that the original data bit sequence is correctly reproduced.
Here, let us now consider conditions under which Viterbi decoding is correctly done. For the correct path to be ultimately chosen, the path metric must be smaller for the correct path than for other, error path every time a survival path is determined. This condition is expressed by the equation (3).
(When a correct bit sequence is “ . . . 000”)ΔM=(M(10)[t−1]+Bb[t])−(M(00)[t−1]+Ba[t])>0(When a correct bit sequence is “ . . . 100”)ΔM=(M(00)[t−1]+Ba[t])−(M(10)[t−1]+Bb[t])>0(When a correct bit sequence is “ . . . 011”)ΔM=(M(11)[t−1]+Bf[t])−(M(01)[t−1]+Be[t])>0(When a correct bit sequence is “ . . . 111”)ΔM=(M(01)[t−1]+Be[t])−(M(11)[t−1]+Bf[t])>0  equation (3)(When a correct bit sequence is “ . . . 001” or “ . . . 110”)
Because determination of a survival path never fails to be performed correctly, a relation of ΔM>0 always exists.
In the equation (3), ΔM is a difference between path metrics of two paths one of which will be the survival path, and the difference is termed SAM. It is necessary that SAM>0 so that any error does not occur, which shows that: the larger SAM becomes, the less error occurs.
As a method for compensating and setting a recording condition by using SAM, Japanese Unexamined Patent Publication No. 151219/2003 (Tokukai 2003-151219) (published on May 23, 2003) (Document 1) discloses a method in which: a first pattern including a coded bit sequence “10” or “01”, a second pattern whose coded bit sequence “10” or “01”, is replaced with “11”, and a third pattern whose coded bit sequence “10” or “01” is replaced with “00” are provided, a first distance between a reproduced signal and the first pattern, a second distance between the reproduced signal and the second pattern, and a third distance between the reproduced signal and the third pattern are obtained, and a recording condition is compensated and set based on a first distance difference D2 between the first distance and the second distance (=the second distance−the first distance) and the second distance difference D3 between the first distance and the third distance (=the third distance−the first distance).
Here, the following describes an example of the method for compensating a recording condition disclosed in Document 1. The first pattern is “111000”, the second pattern is “111100”, and the third pattern is “110000”. At that time, the second pattern is different from the first pattern in that the coded bit sequence “10” is replaced with “11”. The third pattern is different form the first pattern in that the coded bit sequence “10” is replaced with “00”.
FIG. 37 illustrates paths of the first and second patterns on a trellis diagram. The first distance difference D2 between the first distance and the second distance (the first distance is between the reproduction signal and the first pattern and the second distance is between the reproduction signal and the second pattern) corresponds to a path metric difference at a time t+4 in FIG. 37. Further, FIG. 38 illustrates paths of the first and third patterns on a trellis diagram. The second distance difference D3 between the first distance and the third distance (the first distance is between the reproduction signal and the first pattern and the third distance is between the reproduction signal and the third pattern) corresponds to a path metric difference at a time t+3 in FIG. 38.
At that time, when the first pattern is stored, the reproduction signal of the first pattern is more likely to be misidentified as the second pattern as the first distance difference D2 is nearer to 0. Further, when the first pattern is stored, the reproduction signal of the first pattern is more likely to be misidentified as the third pattern as the second distance difference D3 is nearer to 0. When compensation of a record is performed so that the probability at which the reproduction signal of the first pattern may be misidentified as the second pattern is equal to the probability at which the reproduction signal of the first pattern may be misidentified as the third pattern, it is possible to reduce a probability of misidentifying the reproduction signal of the first pattern.
Here, a phenomenon in which the reproduction signal of the first pattern is misidentified as the second pattern when the first pattern is stored is opposite to a phenomenon in which the reproduction signal of the first pattern is misidentified as the third pattern when the first pattern is stored. Therefore, when the distance between the first distance difference D2 and the center 0 is substantially the same as the distance between the difference between −D3 which is the negative value of the second distance difference D3 and the center 0, it is possible to reduce the possibility at which the reproduction signal of the first pattern may be misidentified.
However, there is a case where a DC offset occurs in a reproduction signal to be inputted to a Viterbi decoding circuit for performing Viterbi decoding due to variations of a direct current level caused by deviation in duty of a mark on a disc or to deviation of offset adjustment of an A/D converter for converting the reproduction signal from analog data to digital data. Here, DC offset is a difference between a level of a signal inputted to a Viterbi decoding circuit when there is no reproduction signal and a level of an amplitude center (0) level of an ideal waveform. FIG. 39 illustrates a waveform (full line) resulting from occurrence of DC offset in an ideal waveform (broken line).
FIG. 40 illustrates changes in the first distance difference D2 and −D3 which is the negative value of the second distance difference when the DC offset occurs in the ideal waveform. Ideal values are 1.5 and −1.5 respectively. The values change according to the DC offset.
Document 1 discloses that: when D2 and −D3 are positioned so that the distance between D2 and the center 0 is substantially the same as the distance between −D3 and the center 0, it is possible to reduce the probability of misidentifying the reproduction signal. As such, when the distance from the center 0 changes due to the influence of DC offset, it is impossible to obtain a good recording condition.
Further, there is a case where clock phase deviation occurs in the reproduction signal to be inputted to the Viterbi decoding circuit for performing Viterbi decoding due to misadjustment of offset in a reproduction clock extraction circuit for extracting a reproduction clock required in the A/D converter from the reproduction signal. Here, clock phase deviation is a shift in DC between an ideal sampling phase of PRML and a clock phase. FIG. 41 illustrates a case (black point) where sampling is performed so that clock phase deviation in DC occurs in an ideal sampling phase (white point).
FIG. 42 illustrates changes in the first distance difference D2 and −D3 which is the negative value of the second distance difference when the clock phase deviation occurs in the ideal waveform. The ideal values are 1.5 and −1.5 respectively. The values change according to the clock phase deviation.
Document 1 discloses that: as long as D2 and −D3 are positioned so that the distance between D2 and the center 0 is substantially the same as the distance between −D3 and the center 0, it is possible to reduce the probability of misidentifying the reproduction signal. Therefore, there is a case where a good recording condition cannot be obtained due to the influence of clock phase deviation.
Further, Document 1 discloses an arrangement in which the recording condition is compensated and set by using the first distance difference D2 between the first distance and the second distance (the first distance is between the reproduction signal and the first pattern and the second distance is between the reproduction signal and the second pattern) and the second distance difference D3 between the first distance and the third distance (the third distance is between the reproduction signal and the third pattern). However, it is not so arranged that: first distance differences for each mark length are further classified by a pattern including the first pattern and the values of the first distance differences are compared with each other, so as to detect an even condition of an edge deviation of a recording mark between mark lengths. Further, it is not so arranged that: second distance differences for each mark length are classified by a pattern including the first pattern and the values of the second distance differences are compared with each other, so as to detect an even condition of an edge deviation of a recording mark between mark lengths. Therefore, the edge deviation between mark lengths may occur, with a result that a good recording condition cannot be obtained.